A digital sender is a system designed to obtain documents (for example by scanning), convert the documents to a chosen format and route the formatted document to a desired destination or destinations using an available communication protocol. Digital senders generally support a variety of document types, a variety of data formats, and a variety of communication protocols.
Examples of typical document formats include tagged image file format (TIFF), multipage TIFF (MTIFF), portable document format (PDF), and joint picture experts group (JPEG). Examples of typical communication methods include computer networks and facsimile transmission (fax).
Documents can be classified based on content. For example, text documents typically contain black text on a white background. Formats used to transmit text documents typically are optimized to provide for crisp edges to effectively define characters. Traditional fax is designed to efficiently transmit text (black text on a white background) documents.
Graphics documents typically contain color or grayscale images. Formats used to transmit continuous tone images, for example, continuous tone color photographs, can be very effectively represented using the JPEG format.
Mixed content documents typically include a combination of text and graphic data. These documents often require more specialized solutions because existing formats used for transmission and storage of image data are optimized for use with either black and white text, or with continuous tone images.
The current TIFF specification supports three main types of image data: black and white data, halftones or dithered data, and grayscale data.
Baseline TIFF format can be used to store mixed content documents in black and white (i.e. binary) formats. Baseline TIFF format supports three binary compression options: Packbits, CCITT G3, and CCITT G4. Of these, CCITT G3, and CCITT G4 compression are compatible with fax machines.
Halftoning algorithms, such as error diffusion, can be used to create a binary representation of (i.e. binarize) a continuous tone image. Such an image can be subsequently compressed using CCITT G3, and CCITT G4 compression so they are suitable for fax transmission. However, CCITT G3 compression, and CCITT G4 compression generally do not provide for the desired compression ratios for halftone images. Therefore CCITT G3, and CCITT G4 compression of halftone mixed content documents results in large file sizes and subsequently very long fax transmission times.
A binary representation of an input document can be created by performing a binary threshold operation using a constant threshold for the entire image. However, when CCITT G3/G4 compression is performed on such a document, there is generally unsatisfactory representation of continuous tone document content and color text on color background. Likewise, when halftoning an input document using error diffusion such as Floyd Steinberg, with CCITT G3/G4 compression, this can result in inadequate compressibility of halftone using G3 and G4 (i.e. fax) compression.